1. Field of the Invention
The invention relates to an optical information read/write device in use for recording information signals on an optical information recording medium having improved information density, where information is written on guide grooves as well as regions between two guide grooves adjacent with each other, all guide grooves of the optical recording medium being formed before writing information.
2. Description of the Prior Art
In recent years there have been extensive research and development on optical information recording media to achieve high density recording of such information as image and voice signals. One of the well-known examples is an optical disk. A writable optical disk has guide grooves formed in advance by cutting on a transparent substrate. Information signals are read or written by focusing a laser beam onto these tracks. When a commercially available optical disk is used for writing information signals, signals are normally written on either guide grooves or regions between two guide grooves adjacent with each other (hereinafter referred to as the land), but not on both.
When writing information signals, a laser beam from an optical head is focused by an object lens onto a recording layer in a guide groove. When the intensity of the laser beam is strong, the region the laser beam is focused on undergoes a change in reflectance, and a record mark is formed. The distance between record marks on adjacent guide grooves is the width of the land between them.
When reading information signals, record marks arranged along a guide groove are irradiated by a focused laser beam, which is either reflected or absorbed, thus the intensity of the reflected laser beam varies. Reading of information signals is performed by an optical head detecting the variation of the intensity of the reflected light. The track pitch, that is, the spatial period of guide grooves is about the size of the laser beam spot and set to be 1.6 .mu.m in this explanation. The depth of the guide groove is about one eighth of the wavelength of the read beam in optical length. This depth gives rise to the maximum push-pull signals of reflected light from an optical disk having guide grooves, thus provides the most stable push-pull tracking control.
In order to increase memory capacity of an optical disk it would be natural to write information signals on both guide grooves and lands and increase information track density. In this method, however, the distance between two adjacent information tracks is approximately a half the laser beam spot. Therefore, the beam overlaps the information tracks next to the information track intended to read. Thus, the cross talk for reading becomes significant causing degradation of the read S/N ratio. A paper of SPIE Vol. 1316 Optical Data Storage (1990), pp. 35, "High track density magneto-optical recording using a cross talk canceller" describes a method to reduce the cross talk, in which an optical read apparatus comprises triple-beam optics and a cross talk cancellation circuit. A drawback, however, is that the optics and signal processing circuits become large and complex.
A U.S. Pat. No. 4,423,502 presents a technique to reduce cross talk between adjacent information tracks by using different effective levels for adjacent information tracks. When information is written on both guide grooves and lands, the difference between the effective levels means the depth of the guide grooves. According to the U.S. Patent, when the writing mechanism is to change the reflectivity of the medium, the desirable effective level difference is between .lambda..sub.e /8 and .lambda..sub.e /4 (.lambda..sub.3 is the wavelength of the read laser beam at the recording medium). This method allows one to reduce the cross talk without using the cancellation circuits mentioned above.
This patent, however, mentions nothing about erasable media. We applied the method to an erasable optical disk and found the following problems. (1) When the width of the guide groove is narrower compared to the read beam spot, the beam erases part of marks on the adjacent tracks causing degradation of the read signals. (2) When the width of the guide groove is wider compared to the write beam spot, the write beam tends to wobble from the center line of the guide groove. It causes incomplete erase of old marks when writing in the over-write scheme, thus resulting again in degradation of the read signals. (3) If the thermal conductivities of medium materials are high, or the structure of the medium makes the thermal conductivity high, the heat generated in writing erases part of marks on the adjacent tracks giving rise to degradation of the read signals.